UPDATED 3.27pm: A major diesel spill that poisoned a central North Island's water supply is now being investigated by police.

Members of the public have lodged complaints about  a spill at Tongariro National Park's Turoa Skifield in 2013 which contaminated the Makotuku River and the Raetihi water supply.

Ruapehu Alpine Lifts was prosecuted last year and fined over the incident.

Green MP Eugenie Sage says private companies need to treat the opportunity to operate at national parks as a privilege, not a right.

"They have got to implement the highest standards of safety when it comes to things like diesel and other hazardous substances."

Sage argues the Department of Conservation is under-funded and standards aren't being enforced.

"It was really disappointing that the department had so many [fuel] tanks that didn't [comply to the required standards]."